1. Field of the Invention
The present invention relates in general to methods of fabricating microelectronic structures using EUV (extreme ultraviolet) lithography.
2. Description of the Prior Art
As device generations continue toward the 22-nm node, EUV lithography is expected to be the method of choice to achieve the required critical dimension (CD) targets. One of the main challenges in material design, particularly with resists, is achieving the improvement in line width roughness (LWR) and sensitivity necessary to achieve industry targets. The International Technology Roadrnap for Semiconductors has set targets for 3G LWR at <3 nm and for sensitivity at <10 mJ/cm2. Most currently available resists do not meet both targets and tend to achieve a compromise between LWR and sensitivity.
Numerous attempts have been made to improve resist sensitivity while improving LWR. Among the various approaches is incorporating a photoacid generator (PAG) functionality into the resist polymer. Such methods are limited to the maximum or minimum amount of PAG that must be present in the polymer formulation for optimum lithographic performance. It is also synthetically challenging to obtain the ideal PAG composition for incorporation into a polymer backbone.
In another approach for resist design, halogens have been used in an attempt to increase resist absorbance to reduce LWR and increase sensitivity. This approach is promising, but the resist needs a certain level of transparency to minimize tapering that can occur at the bottom of the resist. Tapering is caused by the difference in light absorption that occurs as photons travel from the top of the resist to the bottom of the resist.
It has also been suggested that resist absorbance and secondary electron generation by EUV irradiation play an important role in acid generation that can improve LWR and sensitivity. However, as discussed above, there are limitations on the increase in absorbance that can be achieved in a resist film before tapering occurs.
There is a need for improved EUV methods that improve LWR while simultaneously having the necessary sensitivity.